The invention relates to improved paper machine stock screens, and more particularly to an improvement in a profiled screen construction which improves the efficiency and effectiveness of the screening operation.
Stock screens are used in the paper making process for aiding in cleaning the stock before it flows to the headbox to be dewatered to form a web. Such stock screens are conventionally tubular in shape, with the stock being admitted near one end of the screen and directed either to the inner surface or the outer surface of the tubular screen. The accepted stock flows through the screen, and the rejected stock including shives, particles, dirt and the like not passing through the screen flows to the other end of the screen to be removed through a reject line. Typically, the accepts are received by an annular chamber, with the annular chamber surrounding the screen in the case where the supply stock is delivered to the inside of the screen and the stock flows outwardly through the screen in the screening operation. In commercial operation, it is desirable that the screen operate under pressure. Stock is pumped into one end of the tubular screen to enter the interior of the screen tangentially, and the pressurized screen will operate under a wide range of stock velocity. For aiding in the rapid flow of acceptable stock through the screen and preventing the fibers from building up on the screen surface, shaped foils are continuously moved around along the surface of the screen.
U.S. Pat. No. 4,855,038 discloses an improved arrangement for generating a turbulence along the screen basket surface providing strong negative and positive pulses and inducing a pulsating flow along the inner surface of the screen.
With the generation of the pulsations, accepted fibers flow through openings in the screen and are captured to flow to a headbox or to a vat. The rejected fibers and impurities, which do not pass through openings in the screen, pass out through a reject outlet.
In working with paper machine pulp screening apparatus, it has been noted that at times variations in throughput of 4 to 5 times magnitude were experienced. The screen baskets are changeable, and changing baskets which were of similar construction resulted in a wide yield variation. Upon further study, it was determined that the characteristic of the inner screening surface of the cylindrical screen had a pronounced effect on the throughput of the screen as well as its durability in reduced breakage and the elimination of the necessity of frequent cleaning.
It is accordingly an object of the present invention to provide an improved screen structure for screening paper machine pulp which can be constructed in accordance with certain parameters, and which yields a surprising increase in throughput of acceptable stock.
A further object of the present invention is to provide an improved screen structure which resists wear and eliminates the need for frequent screen replacement.
A still further object of the present invention is to provide an improved paper machine pulp screen capable of operation without frequent cleaning, avoiding the accumulation of residual material in the openings.
Yet another object of the present invention is to provide an optimum screen profile, which in operation yields consistent performance in throughput, efficiency, and durability.